Effect of Alloying Elements (Ga, Ge, Si) on Pseudoelasticity in Fe 3 Al Single Crystals
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Effect of Alloying Elements (Ga, Ge, Si) on Pseudoelasticity in Fe3Al Single Crystals Hiroyuki Y. Yasuda1,2, Takashi Kase2 and Yukichi Umakoshi2 1 Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, 7-1, Mihogaoka, Ibaraki, Osaka 567-0047, Japan 2 Department of Materials Science and Engineering, Graduate School of Engineering, Osaka University, 2-1, Yamada-oka, Suita, Osaka 565-0871, Japan ABSTRACT Pseudoelasticity in Fe3Al single crystals doped with a small amount of Ga, Ge and Si was investigated focusing on the antiphase boundary (APB) energy and the ordered domain structure. Single crystals of Fe-23at%Al and Fe-21at%Al-2at%X (X=Ga, Ge, Si) were grown by a floating zone method. In Fe-23at%Al single crystals, superpartial dislocations with Burgers vector (b) of 1/4 moved dragging APB during loading, while APB pulled back the superpartials during unloading. This resulted in giant pseudoelasticity regardless of martensitic transformation and the recoverable strain was about 5%. Ga addition was found to be effective in increasing the recovery strain compared with Fe-23at%Al crystals. In contrast, both Ge and Si additions decreased the amount of shape recovery. Stress at which the shape recovery started, was increased by Ga, Ge and Si additions. This means the APB energy increased by the additions, since the surface tension of APB pulling back the superpartials increases with increasing the energy. On the other hand, the frictional stress of the superpartials with b=1/4 increased significantly by Ge or Si doping due to solid solution hardening, though the stress of Ga-doped crystals was almost the same as that of the binary crystals. Higher frictional stress of Ge- and Si-doped crystals made the reversible motion of the superpartials difficult, resulting in the small recovery ratio. Ordered domains with displacement vector (R) of 1/4 in Fe-23at%Al and Fe-Al-Ga alloys were observed to be small, less than 100nm. In contrast, Ge and Si additions increased the domain size to more than 500nm. Since the domain boundaries with R=1/4 played an important role in the individual motion of the superpartials with b=1/4, the fine domain structure was found to be favorable for giant pseudoelasticity in Fe3Al single crystals. Ga addition increased the APB energy following the superpartials and kept the domain size small, resulting in the increase in recovery strain. INTRODUCTION Pseudoelasticity is a phenomenon by which strain is recovered during unloading. A giant pseudoelasticity generally results from a martensitic transformation; stress-induced martensitic transformation during loading and the reverse transformation during unloading [1]. However, D03-ordered Fe3Al single crystals where martensitic transformation never occurred were found to demonstrate pseudoelasticity [2-4]. In Fe3Al with the D03 structure, a superlattice dislocation with Burgers vector (b) of generally dissociates into four superpartial dislocations with b=1/4 bound by antiphase boundary (APB) [5]. In contrast, the superpartia
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